1Lightweight PI-futexes
2----------------------
3 4We are calling them lightweight for 3 reasons:
5 6 - in the user-space fastpath a PI-enabled futex involves no kernel work
7 (or any other PI complexity) at all. No registration, no extra kernel
8 calls - just pure fast atomic ops in userspace.
9 10 - even in the slowpath, the system call and scheduling pattern is very
11 similar to normal futexes.
12 13 - the in-kernel PI implementation is streamlined around the mutex
14 abstraction, with strict rules that keep the implementation
15 relatively simple: only a single owner may own a lock (i.e. no
16 read-write lock support), only the owner may unlock a lock, no
17 recursive locking, etc.
18 19Priority Inheritance - why?
20---------------------------
21 22The short reply: user-space PI helps achieving/improving determinism for
23user-space applications. In the best-case, it can help achieve
24determinism and well-bound latencies. Even in the worst-case, PI will
25improve the statistical distribution of locking related application
26delays.
27 28The longer reply:
29-----------------
30 31Firstly, sharing locks between multiple tasks is a common programming
32technique that often cannot be replaced with lockless algorithms. As we
33can see it in the kernel [which is a quite complex program in itself],
34lockless structures are rather the exception than the norm - the current
35ratio of lockless vs. locky code for shared data structures is somewhere
36between 1:10 and 1:100. Lockless is hard, and the complexity of lockless
37algorithms often endangers to ability to do robust reviews of said code.
38I.e. critical RT apps often choose lock structures to protect critical
39data structures, instead of lockless algorithms. Furthermore, there are
40cases (like shared hardware, or other resource limits) where lockless
41access is mathematically impossible.
42 43Media players (such as Jack) are an example of reasonable application
44design with multiple tasks (with multiple priority levels) sharing
45short-held locks: for example, a highprio audio playback thread is
46combined with medium-prio construct-audio-data threads and low-prio
47display-colory-stuff threads. Add video and decoding to the mix and
48we've got even more priority levels.
49 50So once we accept that synchronization objects (locks) are an
51unavoidable fact of life, and once we accept that multi-task userspace
52apps have a very fair expectation of being able to use locks, we've got
53to think about how to offer the option of a deterministic locking
54implementation to user-space.
55 56Most of the technical counter-arguments against doing priority
57inheritance only apply to kernel-space locks. But user-space locks are
58different, there we cannot disable interrupts or make the task
59non-preemptible in a critical section, so the 'use spinlocks' argument
60does not apply (user-space spinlocks have the same priority inversion
61problems as other user-space locking constructs). Fact is, pretty much
62the only technique that currently enables good determinism for userspace
63locks (such as futex-based pthread mutexes) is priority inheritance:
64 65Currently (without PI), if a high-prio and a low-prio task shares a lock
66[this is a quite common scenario for most non-trivial RT applications],
67even if all critical sections are coded carefully to be deterministic
68(i.e. all critical sections are short in duration and only execute a
69limited number of instructions), the kernel cannot guarantee any
70deterministic execution of the high-prio task: any medium-priority task
71could preempt the low-prio task while it holds the shared lock and
72executes the critical section, and could delay it indefinitely.
73 74Implementation:
75---------------
76 77As mentioned before, the userspace fastpath of PI-enabled pthread
78mutexes involves no kernel work at all - they behave quite similarly to
79normal futex-based locks: a 0 value means unlocked, and a value==TID
80means locked. (This is the same method as used by list-based robust
81futexes.) Userspace uses atomic ops to lock/unlock these mutexes without
82entering the kernel.
83 84To handle the slowpath, we have added two new futex ops:
85 86 FUTEX_LOCK_PI
87 FUTEX_UNLOCK_PI
88 89If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to
90TID fails], then FUTEX_LOCK_PI is called. The kernel does all the
91remaining work: if there is no futex-queue attached to the futex address
92yet then the code looks up the task that owns the futex [it has put its
93own TID into the futex value], and attaches a 'PI state' structure to
94the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware,
95kernel-based synchronization object. The 'other' task is made the owner
96of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the
97futex value. Then this task tries to lock the rt-mutex, on which it
98blocks. Once it returns, it has the mutex acquired, and it sets the
99futex value to its own TID and returns. Userspace has no other work to
100perform - it now owns the lock, and futex value contains
101FUTEX_WAITERS|TID.
102 103If the unlock side fastpath succeeds, [i.e. userspace manages to do a
104TID -> 0 atomic transition of the futex value], then no kernel work is
105triggered.
106 107If the unlock fastpath fails (because the FUTEX_WAITERS bit is set),
108then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the
109behalf of userspace - and it also unlocks the attached
110pi_state->rt_mutex and thus wakes up any potential waiters.
111 112Note that under this approach, contrary to previous PI-futex approaches,
113there is no prior 'registration' of a PI-futex. [which is not quite
114possible anyway, due to existing ABI properties of pthread mutexes.]
115 116Also, under this scheme, 'robustness' and 'PI' are two orthogonal
117properties of futexes, and all four combinations are possible: futex,
118robust-futex, PI-futex, robust+PI-futex.
119 120More details about priority inheritance can be found in
121Documentation/rt-mutex.txt.
122